Now almost every semicon is reputing GaN as best high power voltage technology. GaN is the most practical, lowest loss, power semiconductor material available today. SiC can achieve reasonable performance as a low-loss device, but experts have assured me the performance of fully developed GaN will be two times (2X) better than SiC.
GaN, a wide bandgap semiconductor successfully implemented in optical and high-speed electronic devices, has gained momentum in recent years for power electronics applications. Along with rapid progress in material and device processing technologies, high-voltage transistors over 600 V have been reported by a number of teams worldwide. These advances make GaN highly attractive for the growing market of electrified vehicles, which currently employ bipolar silicon devices in the 600–1200 V class for the traction inverter. However, to capture this billion-dollar power market, GaN has to compete with existing IGBT products and deliver higher performance at comparable or lower cost. This paper reviews key achievements made by the GaN semiconductor industry, requirements of the automotive electric drive system and remaining challenges for GaN power devices to fit in the inverter application of hybrid vehicles.
Today, the need of electric power is continuously increasing and represents a global concern. Many discrete power electronic devices are used in the power modules for the transmission and the conversion of electric power. For these devices, a reduction of the static and dynamic losses can directly result in the overall lowering of power consumption of the system. Also the next generation of high-speed communication devices are becoming key technologies for network communication, requiring increasing operating frequency associated with portability and convenience.
Since the production of the first silicon transistor by Gordon Teal at the Texas Instruments [22], silicon (Si) has assumed a central role in the development of semiconductor devices. Recently however, Si technology is approaching the theoretical limits imposed by the material properties, in terms of maximum operation power, frequency and temperature.
In this general context, the research activities in modern electronics clearly hint towards new materials able to satisfy the specific need of higher operation frequency, higher output power and higher operation voltages.
The use of wide bandgap (WBG) materials can be considered as the best solution to meet the requirements of modern power electronics. In fact, WBG semiconductors such as silicon carbide (SiC) and gallium nitride (GaN), have been known to exhibit superior electrical characteristics compared to Si because of their inherent advantages such as high electron mobility, higher breakdown field strength and larger energy bandgap. Indeed, significant advances have been accomplished in the recent past in the growth, doping and devices processing technologies of wide bandgap materials, opening new avenues in semiconductor electronics in general, and high power electronics in particular.
While SiC has shown in the last two decades the most progress with respect to improved material quality and device processing, GaN is still affected by several challenging aspects, related both to specific material issues and to a non-conventional technology if compared to Si.
Wide-band Gap Devices: Technology Benefits & Challenges
Wide-band gap devices (WBG) can operate at approximately twice the voltage of Si-based semiconductors as they have a higher energy band-gap than Si, around 3 times of Si, which means higher energy is needed to excite electrons from the valence band into the conduction band. They also have higher thermal conductivity (heat is more easily transferred from the device and is thus less likely to cause heat-related problems in other devices) and can operate at higher frequencies. These devices benefit from low conduction resistance (i.e. Rds-on) as well as lower capacitive charges. Since wide bandgap technologies have high critical fields, they can handle high voltages much better than Si. These properties lead to WBG devices with higher thermal and radiation durability. The critical junction temperature of Si devices which makes it uncontrollable is around 150°C while it is around 900°C for WBG devices. These semiconductors are also much lighter and smaller than Si-based devices of comparable ratings.
• Two different Wide band gap technologies have been the topic of research now. Those are SiC (Silicon Carbide) and GaN (Gallium Nitride) based devices. As already discussed earlier, there are several benefits over Si-based devices;
• Much smaller switching losses, leading to higher efficiency
• Ability to operate at higher temperatures without much change in electrical properties, leading to better reliability
• Smaller heat sink because of lower losses
• High frequency of operation allows smaller filters, which leads to light and compact packaging
Currently, SiC is considered to have the best trade-off between properties and commercial maturity. GaN can offer better HF and HV performances, but the lack of good quality large area substrates is a disadvantage for vertical devices. GaN presents a lower thermal conductivity than SiC and allows forming 2DEG heterojunctions (InAlGaN alloys) grown on SiC or Si substrates. So it is essentially a race between SiC and GaN in terms of performance and cost.
There are also some potential challenges associated with the wide band gap device of today. Those are:
a. High cost associated with WBG devices
b. Normally-on GaN and SiC devices
c. In order to migrate to wide bandgap technologies, gate driver circuits have to be replaced as well. For commercially available SiC MOSFETs, the recommended on-voltage is 20 V which is higher than the Si technology. Hence, replacing the gate driver can impose extra cost.
d. GaN-on-SiC technologies demonstrate very good figures for efficiency and maximum switching frequency among the available technologies. However, these switches are not cost effective for power converter applications.
e. In low voltage applications, GaN-on-Si provides good efficiency and switching characteristics at a reasonable cost. However, these switches are not commercially available at higher voltage levels (currently 650V).
f. Though SiC MOSFETs are available in medium voltage levels, currently, the economic aspects prevent integration of this technology in commercial transportation electrification.
Sic/GaN devices from Manufacturers
Lower power SiC diodes are commercially available for long time, but research was constrained by a severe cost penalty of the SiC starting material. Boosted by the widespread use of GaN in optoelectronics – GaN is the starting material for blue and green emitting (laser) diodes – researchers started building devices for radar and RF for military and aerospace applications, but recent advances in the challenging epitaxy of GaN on silicon substrates bear the promise to significantly reduce costs down to silicon levels, triggering investigation on GaN use for high volume markets among which the power conversion.
Currently, commercialization of these technologies are restricted by the availability of the switches as well as economic concerns. Manufacturing low defect (defects due to discoloration and dislocation effects) Silicone-Carbide (SiC) substrates are expensive and not compatible with conventional fabrication equipment. For this reason, SiC technology carries a high price tag. On the other hand, high speed Gallium-Nitride (GaN) switches are manufactured on SiC substrate. These switches are costly and not suitable for power conversion applications. For this reason, GaN on Si switches have been introduced and commercialized for applications in power electronics converters. These switches benefit from fast switching times of GaN technology while maintaining a comparable price with Si technology.
Several manufacturers are manufacturing WBG switches these days. For example, BJT from Fairchild, GeneSiC (SJT), APEI(500 A WBGS), Microsemi, JFET from Infineon and MOSFET from Cree are some of the examples. For the voltage rating, most of the announced devices are 1200V, which meet the requirements for 600V DC bus applications in EVs. Only recently, Fairchild and GeneSiC have announced their high temperature SiC BJTs with maximum junction temperature of 250°C which could be an ideal choice for high temperature applications such as traction power converters. Besides the discrete package transistors, manufactures such as Cree and Rohm, have built up power modules with SiC transistors and SiC Schottky diodes for higher power applications.
Production Challenges
The production process for the Wide band gap devices is significantly slower than that of silicon crystals for standard semiconductors. Producing the crystals to fabricate the wafers requires temperatures in excess of 2000° C for SiC. The material does not melt; it sublimates and the crystals must then be condensed from the vapors. The largest wafer size currently commercially available for this material is four inches (100mm). While the capability to produce six-inch wafers exists with Cree, market demand is currently insufficient to warrant production.
Gallium nitride (GaN) is produced by condensing the GaN vapors (usually created by combining the vapors of a gallium compound with nitrogen or ammonia vapors) into crystals or by condensing the GaN in layers directly onto the substrate (typically SiC or sapphire, though success in depositing GaN directly onto Si substrates has recently been reported in R&D projects). According to Cree, these processes are prone to producing crystals with substandard lattice structures. The crystals produced are smaller than either Si or SiC crystals, thus driving up the cost of producing bulk materials.
In terms of higher-voltage applications relevant to drive-trains, the result is that market demand for emerging WBGS modules is low because their manufacturing cost is high, in part due to the unavoidably complex manufacturing processes. Low market demand in turn leads to especially high cost-premiums given the large capital expenditures that are spread over relatively low-volume production runs.
